In current large-scale networks, information flows through a series of nodes in the network from one location or site to another. As the network grows, more and more transmission lines may be added to handle the heavy traffic flow between nodes. FIG. 1 illustrates a related art system 100 that has east 110 and west 140 terminals. The east 110 and west 140 terminals communicate via lines (e.g., lines 152, 154, 156 and 162, 164, 166) that run between the terminals (e.g., optical fiber pairs 156 and 166, 164 and 154, 152 and 162. East 110 and west 140 terminals can be located a significant distance apart.
Accordingly, line amplifier nodes (e.g., 120, 130) can be interposed between the terminals (e.g., every 40-80 kilometers) to compensate for the signal loss in the transmission medium (e.g., optical fiber) by amplifying the signal. Additionally, associated dispersion compensation modules (e.g., DCMs 122, 128, 132 and 138) can be added to correct for the signal degradation caused by the transmission medium (e.g., dispersion in the optical fibers). Further, to increase the bandwidth available each east and west line can have a plurality of channels communicated on separate bands (e.g., red and blue signals), as is known in the art.
If the bandwidth provided by both bands is not needed for each direction, a single fiber can be used to carry both directions using one band for the east direction and one band for the west. There are several implementations of a bidirectional optical communication on a single fiber. These single-fiber implementations use counter-propagating channels from two non-overlapping bands of the optical spectrum. Although this system configuration can reduce the number of fibers for bidirectional communication, it can further complicate design of the amplifier nodes. For example, in one solution the red and blue signals are separated at respective inputs, routed through separate amplifiers, and then recombined at the respective outputs. However, this configuration still requires two amplifiers per node, one for each direction.
FIG. 2 illustrates an alternative configuration according to the related art for designing a bidirectional amplifier as disclosed in U.S. Pat. No. 6,018,404 entitled “Bidirectional Optical Telecommunication System Comprising a Bidirectional Optical Amplifier.” The bidirectional amplifier configuration includes four wavelength (λ) selective optical couplers 221-224, one unidirectional optical amplifying unit 220, two optical connectors 206, 207 and portions 225, 226, 227, 228 of passive optical fiber. The components are coupled with each other to form an optical bridge connection. This configuration uses only one amplifier 220. However, it does not achieve a useful gain when the typical 40 dB optical return loss is taken into consideration.
As illustrated in FIG. 2, two signals λ1 and λ2 pass along a fiber, in opposite direction. The fiber is connected via connectors 206 and 207. Wavelength selective couplers 221-223, split the signals so that they enter the optical amplifier 220 from the same direction, are amplified, and then split by the wavelength couplers 221, 223, and 224 to continue their travel along a common fiber in opposite directions. In the operation of the system described, some of the signals are leaked in the wrong direction (e.g., leakage of amplified signal λ1 254 through coupler 224 that is reflected back 252 from coupler 221 to connector 206), which is called optical return loss (ORL). The general formula that governs the return loss is:Preturn=Pin+G−2Lisolation  (1)ORL=Pin−Preturn,  (2)where, P is power, G is amplifier gain, Lisolation is isolation loss through the device and ORL is optical return loss. Typically, Lisolation for conventional filters, couplers and the like is 20 dB. Accordingly the maximum gain Gmax calculated for a four module system is:Gmax=2Lisolation−ORL=2Lisolation−40=0  (3)Applying this general equation to the configuration illustrated in FIG. 2 yields:Preturn=Pin+G−L224−L221; andGmax=L224+L221−ORL=20+20−40=0  (4)where L224 and L221 are the isolation losses through those corresponding devices. As determined in the foregoing sections, Gmax should be set to zero to avoid excessive amplified signal leakage. Additionally, the four-port configuration also suffers from poor performance when there is a fiber break, which can result in a 14 dB back-reflection.